Pincer-ligated nickel hydridoborate complexes: The dormant species in catalytic reduction of carbon dioxide with boranes

Article abstract of DOI:10.1021/ic300587b

Nickel pincer complexes of the type [2,6-(R₂PO) ₂C₆H₃]NiH (R = ^tBu, 1a; R = ⁱPr, 1b; R = ^cPe, 1c) react with BH₃·THF to produce borohydride complexes [2,6-(R₂

Full text of DOI:10.1021/ic300587b

Article
pubs.acs.org/IC
Pincer-Ligated Nickel Hydridoborate Complexes: the Dormant
Species in Catalytic Reduction of Carbon Dioxide with Boranes
Sumit Chakraborty, Jie Zhang, Yogi J. Patel, Jeanette A. Krause, and Hairong Guan*
Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, United States
S
* Supporting Information
ABSTRACT: Nickel pincer complexes of the type [2,6-(R2PO)2C6H3]NiH (R = tBu,
1a; R = iPr, 1b; R = cPe, 1c) react with BH3·THF to produce borohydride complexes
[2,6-(R2PO)2C6H3]Ni(η2-BH4) (2a−c), as confirmed by NMR and IR spectroscopy,
X-ray crystallography, and elemental analysis. The reactions are irreversible at room
temperature but reversible at 60 °C. Compound 1a exchanges its hydrogen on the
nickel with the borane hydrogen of 9-BBN or HBcat, but does not form any observable
adduct. The less bulky hydride complexes 1b and 1c, however, yield nickel dihydrido-
borate complexes reversibly at room temperature when mixed with 9-BBN and HBcat.
The dihydridoborate ligand in these complexes adopts an η2-coordination mode, as
suggested by IR spectroscopy and X-ray crystallography. Under the catalytic influence of 1a−c, reduction of CO2 leads to the
methoxide level when 9-BBN or HBcat is employed as the reducing agent. The best catalyst, 1a, involves bulky substituents on the
phosphorus donor atoms. Catalytic reactions involving 1b and 1c are less efficient because of the formation of dihydridoborate
complexes as the dormant species as well as partial decomposition of the catalysts by the boranes.
The literature has suggested many possible scenarios for the
reaction of a transition metal hydride complex with a borane. If
the metal center possesses a vacant coordination site, activation
of the borane B−H bond leads to the formation of a σ-borane
complex (structure A in Figure 1),4,5 or a boryl dihydride com-
plex (structure B, endo or exo isomer) as the oxidative addition
product.6 Abstraction of H− by a Lewis acidic boron center
gives rise to a metal complex bearing either a bidentate (struc-
ture C)7 or a monodentate hydridoborate ligand (structure D).8
Evolution of H2 with concomitant formation of a metal-boryl
species (structure E) is another possibility.9 For the reactions
involving HBcat, degradation of the borane by metal hydrides to
[B(cat)2]− has been previously observed.10
Prior to our work, there have been two reports pertaining
to the reaction of a pincer-ligated nickel hydride complex and
a borane, although the motivations for these studies are quite
different from ours. To demonstrate that a Ni−BR′2 moiety
is capable of transferring boron to organic molecules, the
Mindiola group has synthesized a PNP-pincer nickel boryl
complex from the corresponding nickel hydride and HBcat (eq 2).9b
INTRODUCTION
■
We have recently reported an efficient nickel system for the catalytic
reduction of CO2 to the methoxide level under mild conditions
using catecholborane (HBcat) as the reducing agent (eq 1).1
On the basis of NMR studies and density functional theory
(DFT) calculations,2 we have proposed a mechanism
composed of three cycles (Scheme 1), of which each cycle
decreases the formal oxidation state of carbon by 2 followed by
the consumption of 1 equiv of HBcat to regenerate the active
nickel hydride species. The computed free-energy profile of the
entire process initially suggested to us that the reaction of
[Ni]H with HCOOBcat (the first step of Cycle II) would cross
the highest kinetic barrier.2 We had therefore anticipated that
t
replacing the bulky Bu groups in the nickel pincer complex
with smaller alkyl groups such as Pr and cPe (cPe =
i
cyclopentyl) would accelerate the turnover-limiting step,
leading to more efficient catalysis. Experimental results have
shown that, on the contrary, analogous nickel complexes with a
more accessible hydride moiety are, in fact, inferior catalysts.3
This discrepancy may be reconciled by invoking an off-the-
catalytic-loop adduct generated from the nickel hydride and
HBcat (highlighted in red in Scheme 1). If this is the case, the
structure of this adduct, the extent to which it forms, and the rate
at which it releases the nickel hydride back to the catalytic cycles
are all important in better understanding the catalytic system.
Received: March 20, 2012
Published: May 16, 2012
© 2012 American Chemical Society
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Scheme 1. Proposed Mechanism for Nickel-Catalyzed Reduction of CO2 with HBcat
the electronic difference between analogous PCP- and POCOP-
pincer complexes is not trivial because the influence of electron-
withdrawing oxygen atoms on the phosphorus donors may be
offset by the increased O→C(aryl) π-donation.15 Nevertheless,
electrochemistry studies of (iPrPOCOP)NiBr and (iPrPCP)NiBr
carried out by the Zargarian group have also suggested that the
POCOP-pincer complex is more electron deficient.16 From the
steric point of view, the O-linkages in a (RPOCOP)M system often
impose a more obtuse P−M−P angle than the related (RPCP)M
system, and consequently the metal center supported by a
POCOP-pincer ligand is sterically more accessible.17 This is
certainly the case for nickel hydride complexes; the P−Ni−P angle
of (tBuPOCOP)NiH18 [166.26(3)°] has been found to be smaller
than that of (tBuPCP)NiH19 [173.54(3)°]. We thus suspected that
the more electrophilic and more exposed nickel center of
(tBuPOCOP)NiH would interact with BH3·THF more strongly
than (tBuPCP)NiH, allowing us to isolate the product and fully
characterize it. Guided by this hypothesis, we treated a toluene
solution of (tBuPOCOP)NiH (1a) with a slight excess of BH3·THF
at 0 °C, after which an immediate color change from yellow to
orange was noticed. The color of the reaction mixture remained
orange when warmed up to room temperature. Removal of the
volatiles under vacuum did not appear to reverse the reaction
either, evidenced by the persistent orange color throughout the
Figure 1. Possible products from the reaction of a transition metal
hydride complex with a borane.
In a study of hydride affinity of BH3 toward metal hydrides, Peruzzini
and co-workers have recently investigated the interaction between a
PCP-pincer nickel hydride and BH3·THF, which yields a nickel boro-
hydride complex as suggested by the NMR spectroscopy (eq 3).8
On the basis of DFT analyses, they have proposed a ground-
state η1-BH4 structure, along with the η2-BH4 structure being
the transition state that is responsible for the exchange between
the bridging and terminal hydrogens. However, isolation of this
nickel borohydride complex in the solid-state form has not
been accomplished, mainly because of facile dissociation of BH3
at temperatures that are convenient for workup.
1
evaporation. The H NMR spectrum of the isolated material in
C6D6 displays a broad quartet at −0.45 ppm with an intensity
ratio of approximately 1:1:1:1, indicative of B−H coupling. The
resonance integrates as four hydrogens with respect to one pincer
unit. The boron resonance of the compound appears at −35.9 ppm
as a quintet in 11B NMR, which collapses to a singlet in
11B{1H} NMR. The NMR data suggest that the adduct from
In this paper, we will show unique reactivity of our POCOP-
pincer nickel hydride complexes toward different boranes includ-
ing BH3·THF, 9-BBN, and HBcat (eq 4).
(
tBuPOCOP)NiH and BH3·THF is a typical borohydride
complex.20 Related complexes 2b and 2c were synthesized in
good yield from hydrides 1b and 1c, respectively (eq 5).
By varying the alkyl substituents on the phosphorus donor
atoms of the pincer ligand, we will address how the size of the
substituents impacts the reactivity of the nickel hydride com-
plexes, as well as its relation with the efficiency of the nickel
hydrides in catalyzing the reduction of CO2 with boranes.
RESULTS AND DISCUSSION
■
To further support the structural assignment, we prepared
Reactions of Nickel Hydride Complexes with BH3·THF.
One obvious question is, how different could the reactivity
toward boranes be for the POCOP-pincer complexes?
The ν(CO) stretching frequencies of (tBuPOCOP)Ir(CO)11
[1949 cm−1] and (tBuPCP)Ir(CO)12 [1928 cm−1] seem to support
the notion that the POCOP-pincer ligand makes the metal center
more electron deficient.13 However, as suggested by Goldman
and Krogh-Jespersen, the difference in ν(CO) values may
reflect electrostatic effects rather than the extent of Ir to CO π-
back-donation.14 They have also pointed out that comparing
2a−c through an alternative route involving the displacement
of Cl− by BH4 from nickel chloride complexes 3a−c (eq 6).
−
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a
Table 1. Selected Infrared Frequencies (cm−1) of 2a−c (in Solid State)
2a
2a-D4
2b
2b-D4
2c
2c-D4
ν(B−Ht) or ν(B-Dt)
ν(B−Hb) or ν(B-Db)
BH2 deformation
2412 (s)
2397 (s)
1809 (m)
1749 (m)
1692 (m)
2406 (s)
2382 (s)
1810 (m)
1753 (m)
1697 (m)
2387 (s)
2370 (s)
1800 (m)
1745 (m)
1688 (m)
2274 (w, sh)
1947 (br)
1961 (br)
2068 (br)
1886 (w)
1799 (br)
1122 (s)
1144 (s)
a
Abbreviations for the intensities: s, strong; m, medium; w, weak; sh, shoulder; br, broad.
The substitution reactions of 3b and 3c at room temperature
are completed within 12 h; however, for the sterically more
crowded compound 3a, a prolonged reaction time of 3 days is
needed. In any case, the NMR spectra of the product are
identical to those obtained from the hydride route. The success
of both synthetic methods implies that the POCOP-pincer
nickel borohydride complexes are thermally stable. At room
temperature, they can survive vacuum conditions for at least
24 h, and remarkably, both solid and solution samples of these
complexes can be handled in air without noticeable
decomposition. In strong contrast, the reaction of (tBuPCP)-
NiCl with NaBH4 have been previously shown to generate
has shown two isomeric structures: one contains two trans
η1-BH4 ligands, while the other isomer is ionic with an η2-
BH4 ligand and a BH4 counterion.27 For a dinuclear Ni(II)
−
complex, BH4 bridges two nickel centers as a μ1,3 ligand.28
−
In our case, X-ray structure determination of 2a−c reveals an
η2-BH4 ligand (Figures 2−4), which is consistent with the IR
(
tBuPCP)NiH as the isolable product,21 with (tBuPCP)Ni(BH4)
being the transient intermediate that is only observed in
solution by NMR spectroscopy.8 As discussed earlier, the PCP-
pincer system bears a less electrophilic and sterically more
crowded nickel center, therefore disfavoring the formation of
the borohydride complexes.
−
The coordination mode of BH4 was first established by IR
spectroscopy. Attenuated total reflectance IR spectra of the
solid samples of 2a−c show two strong bands within the range
of 2370−2420 cm−1, which are attributed to terminal
hydrogen−boron stretch [ν(B−Ht)]. The bridging boron−
hydrogen stretching bands [ν(B−Hb)] are very broad but
located in the region 1790−2070 cm−1. Compounds 2b and 2c
also have a strong band at 1144 cm−1 and 1122 cm−1,
respectively, which are assigned as the BH2 deformation mode.
However, such a band is absent in 2a, or more likely obscured
by the bands associated with the pincer framework. The IR
spectra of deuterium-labeled 2a−c (synthesized from 3a−c and
NaBD4) exhibit the predicted terminal deuterium-boron
stretching bands from the known ν(B−H)/ν(B-D) ratio of
around 1.35.22 All these data, as summarized in Table 1, are
consistent with transition metal complexes bearing a bidentate
Figure 2. ORTEP drawing of [2,6-(tBu2PO)2C6H3]Ni(η2-BH4) (2a)
at the 50% probability level. Hydrogen atoms except those attached to
boron are omitted for clarity.
studies described above. As shown in Table 2, the B−H lengths
and H−B−H angles agree with a tetrahedral geometry for the
boron, when factored in with relatively large errors in locating
the positions of hydrogen atoms by X-ray diffraction.
Compared to the reported Ni−H length of 1.37(3) Å for
1a,18 the Ni−H lengths in 2a [1.78(3) Å and 1.85(3) Å] are
substantially elongated. The Ni···B distances in 2a−c
[2.180(3)−2.214(3) Å] are similar to those in trans-NiH(η2-
or η2-BH4 ligand.20,23 Transmission-IR spectra of 2a−c in
−
CH2Cl2 reproduce the major IR features with bands being
slightly shifted from those observed for the solid samples,
suggesting that the η2-coordination mode is also the ground-
state structure in solution. The magnetic equivalence of
1
24
BH4)(PCy3)2 [2.201(8) Å], (triphos)Ni(η2-BH4)26 [2.24 Å]
terminal and bridging hydrogens observed in the H NMR
and [Ni(cyclam)(η2-BH4)]+[BH4]− [2.202(6) Å],27 but sig-
nificantly longer than those in Tp*Ni(η3-BH4) [2.048(5) Å]25
as well as Mindiola’s PNP-pincer nickel boryl complex
[1.9091(18) Å].9b
spectra does suggest that these hydrogens are rapidly
exchanged on the NMR time scale, possibly through an
intermediate involving an η1- or η3-BH4.
Among the known nickel borohydride complexes, very few
have been characterized by X-ray crystallography.24−28 The
Loss of BH3 from Nickel Borohydride Complexes.
Although 2a−c are incredibly robust at ambient conditions,
they do lose BH3 under forcing conditions, particularly in the
case of 2a with a relatively crowded nickel center. For instance,
heating a solid sample of 2a under a dynamic vacuum at 60 °C
for 24 h produces 1a with a 17% conversion. In contrast, 2b
and 2c remain intact under the same conditions. An alternative
24
−
BH4 ligand in both trans-NiH(BH4)(PCy3)2 [Cy = cyclo-
hexyl] and (triphos)Ni(BH4)26 [triphos = MeC(CH2PPh2)3]
coordinates to nickel in an η2 fashion, whereas the same ligand
in Tp*Ni(BH4)25 [Tp* = hydrotris(3,5-dimethylpyrazolyl)-
borate] adopts an η3 coordination mode. A recent study of
Ni(cyclam)(BH4)2 [cyclam = 1,4,8,11-tetraazacyclotetradecane]
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a
Table 2. Selected Bond Lengths (Å) and Angles (deg)
2a
2b (A)
2b (B)
2c
Ni−Hb
B1−Hb
B1−Ht
1.78(3)
1.85(3)
1.17(3)
1.22(3)
1.04(3)
1.06(3)
1.901(2)
2.2046(7)
2.2025(7)
2.214(3)
1.77(4)
1.78(5)
1.60(3)
1.86(3)
1.14(3)
1.28(3)
1.12(3)
1.17(3)
1.892(2)
2.1475(6)
2.1473(6)
2.180(3)
1.87(4)
1.87(5)
1.10(5)
1.15(5)
1.19(5)
1.15(5)
1.11(5)
1.13(5)
1.16(5)
1.16(5)
Ni−C1
Ni−P1
Ni−P2
Ni···B1
1.898(4)
2.1514(13)
2.1490(13)
2.187(5)
1.900(4)
2.1643(13)
2.1581(13)
2.189(5)
P1−Ni−P2
C1−Ni−P1
C1−Ni−P2
Hb−Ni−Hb
Hb−B−Hb
Ht−B−Ht
163.03(3)
81.65(8)
81.44(8)
65(1)
163.08(5)
81.28(13)
81.98(13)
63(2)
163.14(5)
82.15(12)
81.60(12)
63(2)
161.49(3)
81.96(7)
81.84(7)
67(1)
110(2)
113(3)
112(3)
105(2)
Figure 3. ORTEP drawing of [2,6-(iPr2PO)2C6H3]Ni(η2-BH4) (2b) at
the 50% probability level. Hydrogen atoms except those attached to
boron are omitted for clarity. (Two independent molecules were
found in the crystalline lattice; only one is shown here).
109(2)
117(3)
115(3)
113(2)
a
Two independent molecules were found in the crystalline lattice of
2b, which are indicated here as molecules A and B.
Evidently, the bulky tBu groups in 2a not only facilitate the loss
of BH3, but also shield the P−O bonds preventing potential
decomposition reactions.
Reduction of CO2 with Nickel Borohydride Com-
plexes. Given the fact that 2a−c can dissociate to form nickel
hydrides and a borane (B2H6), we sought to explore the
possibility of reducing CO2 with these borohydride complexes
following mechanistic steps analogous to those outlined in
Scheme 1. When exposed to 1 atm of CO2 at room
temperature, none of the borohydride complexes show any
reactivity. However, at 60 °C, 2a is fully converted to a nickel
formate complex 4a over a period of 8 h (eq 8).
Figure 4. ORTEP drawing of [2,6-(cPe2PO)2C6H3]Ni(η2-BH4) (2c)
at the 50% probability level. Hydrogen atoms except those attached to
boron are omitted for clarity (One of the cyclopentyl rings shows
some disorder; a two-component model is presented for C24/C25
with 55% major occupancy).
1
The H NMR spectrum of the reaction mixture also reveals
several resonances in the range of 3.33−3.62 ppm, suggesting
that CO2 is reduced to (CH3O)3B and other methoxyboryl
species.30 Under the same conditions, 2b and 2c are much less
reactive toward CO2, producing only 5−8% of the correspond-
ing nickel formate complexes even after 48 h. The amounts of
methoxyboryl species are negligible in both cases. The results
here parallel the trend observed in BH3 decomplexation
experiments as described above; a pincer ligand with smaller
way to exclude BH3 from a borohydride complex is by adding
NEt3 to form a stable BH3·NEt3 adduct (eq 7).
−
phosphorus substituents allows nickel to bind to BH4 more
tightly, leading to less efficient release of BH3 for subsequent
reactions.
Such a reaction with 2a at 60 °C yields 1a cleanly and nearly
quantitatively within 4 h. The decomplexation of BH3 from 2b
and 2c, on the other hand, are much more sluggish. After 24 h
at 60 °C, approximately 40% of 2b and 2c are converted to the
corresponding nickel hydride complexes. In addition, side
products start to form, as indicated by 31P{1H} NMR
spectroscopy; however, identifying these species is impossible
because of their instability and the complexity of the NMR
spectra. Our previous work on related pincer complexes
has suggested that the P−O bonds are vulnerable to attack
by a base, resulting in breakdown of the pincer backbone.29
Reactions of Nickel Hydride Complexes with 9-BBN
[9-BBN = 9-Borabicyclo(3.3.1)nonane)]. We have recently
shown that at room temperature in the presence of 9-BBN (as a
dimer), the formate moiety of 4a can be reduced to B-methoxy-
9-BBN.3 Because the hydride species 1a is cleanly produced in
this process and because 1a is known to react with CO2 to
reform 4a, 9-BBN could be a suitable reagent for the catalytic
reduction of CO2, only if its interaction with 1a does not play a
detrimental role. This prompted us to investigate the reactions
of nickel hydride complexes 1a−c with 9-BBN. We first focused
on the reactivity of 1a, which shows no color change upon
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mixing with a half-equivalent of 9-BBN dimer in C6D6. Both 1H
NMR and 31P{1H} NMR spectra of the mixture confirm no net
reaction. It should be noted, however, that the deuterium in
unreacted 1c remained present in the solution. On the basis of
the large one-bond coupling constant of 323.6 Hz and the
splitting pattern, we hypothesized that 9-BBN could also attack
the more exposed pincer backbone to release (cPe)2PH from
the nickel pincer complex as a borane adduct (cPe)2PH·(9-
BBN). This was confirmed by examining the 1H NMR
spectrum of the 2:1 mixture of (cPe)2PH and 9-BBN dimer
in toluene-d8. In addition, this adduct exhibits a 31P resonance
at −6.80 ppm and a 11B resonance at −18.1 ppm, both of which
are present in the corresponding NMR spectra of the 1c/9-
BBN reaction. Despite the complexity due to the side reaction
as well as the unreacted hydride 1c, the IR spectrum of the
reaction mixture in toluene clearly indicates no ν(B−Ht) bands.
Therefore, the dihydridoborate ligand in the major species 5c is
expected to adopt an η2-coordination mode, in a similar fashion
(
tBuPOCOP)NiD exchanges readily with the BH proton of 9-
BBN dimer, indicating some degree of interaction between the
two species. The intermediate for this exchange process could
involve any of the structures A−D shown in Figure 1.
The sterically less demanding hydride 1b behaves drastically
differently. Its yellow solution in C6D6 turns deep red once
mixed with a half-equivalent of 9-BBN dimer. The 31P{1H}
spectrum shows two resonances (both singlets) at 206.55 ppm
and 200.80 ppm in a ratio of 22:78. The minor peak cor-
responds to the unreacted 1b while the major one suggests a
new species 5b being formed. The most characteristic hydrogen
resonance of 5b is a broad singlet at −2.22 ppm (Δν1/2 = 148 Hz
at 22 °C), which integrates as two hydrogens per pincer
molecule. Replacing the NMR solvent with toluene-d8 allowed
us to examine the reaction in a wider temperature range. From
−50 to 40 °C, the ratio between 1b and 5b is nearly invariant
but the hydride resonance of 5b becomes much sharper at
lower temperature. Above 40 °C, both hydride and phosphorus
resonances of 1b and 5b are significantly broadened and
eventually coalesce at about 70 °C. The room-temperature 11B
NMR spectrum shows a singlet at 27.8 ppm for the unreacted
9-BBN31 and a singlet at −10.9 ppm presumably for 5b. The
negative chemical shift is consistent with a four-coordinate
boron center,32 and 5b is thus best described as a dihydrido-
borate complex. The coordination mode of the dihydridoborate
ligand is likely to be η2, because the IR spectrum of the reac-
tion mixture in toluene shows no bands in the ν(B−Ht)
region. Such a coordination mode for [H2-9-BBN]− has also
been commonly observed in other transition metal sys-
tems.7c−f,10b,32b,33 The fact that 1b, 5b, and 9-BBN coexist in
solution with no change in ratios over time suggests that the
reaction is under an equilibrium condition (eq 9).
−
as the BH4 ligand in 2a−c.
Fortunately, X-ray quality single crystals were successfully
grown from a toluene solution of 5c (generated in situ) layered
with pentane at −30 °C. When the crystals were left dry, their
deep red color faded immediately to yellow, implying the
formation of the nickel hydride 1c owing to the dissociation of
9-BBN. Extra care was thus taken to ensure that the crystals
made contact with a small amount of mother liquid, and then
were quickly covered with Paratone oil prior to X-ray
diffraction at low temperature (−123 °C). To our knowledge,
5c is the first crystallographically characterized nickel 9-BBN
complex. As shown in Figure 5, dihydridoborate acts as a
The Keq value at 22 °C was determined to be 13.1 4.5 M−1/2
from 31P{1H} NMR integrations and the initial concentrations
of 1b and 9-BBN.34 The entropy loss due to the formation of
the adduct 5b is presumably compensated by the entropy
gained from the dissociation of 9-BBN dimer, which explains
the virtual insensitivity of the equilibrium constant toward
temperature change.
We reasoned, based on steric grounds, that 1c might have a
slight advantage over 1b in promoting the formation of the
related dihydridoborate complex. Indeed, the reaction of 1c
with 9-BBN in toluene-d8 generates 5c as the major species
with only 5% (based on 31P{1H} NMR) of unreacted 1c. The
hydride resonance of 5c appears at −1.87 ppm as a broad
singlet (Δν1/2 = 107 Hz at 22 °C) and the phosphorus and
boron resonances are found at 187.72 ppm and −11.7 ppm in
31P{1H} and 11B NMR spectra, respectively. One interesting
observation made during the reaction of 1c, however, was that
after several hours, a small amount (<5%) of a second new
species was produced, as first suggested by a proton resonance
centered at 4.14 ppm as a doublet of doublet of triplets (J =
323.6, 15.2, and 3.6 Hz). The amount of this new species did
not increase appreciably over a period of 36 h, and the
Figure 5. ORTEP drawing of [2,6-(cPe2PO)2C6H3]Ni(μ-H)2[B-
(C8H14)] (5c) at the 50% probability level. Hydrogen atoms except
those attached to boron are omitted for clarity (One of the cyclopentyl
rings shows some disorder; a two-component model is presented for
C25 with 70% major occupancy). Selected bond lengths (Å) and
angles (deg): Ni−H 1.71 (3) and 1.77 (2), B1−H both 1.20 (3), Ni−
C1 1.901(2), Ni−P1 2.1570(8), Ni−P2 2.1586(8), Ni···B1 2.207(3),
P1−Ni−P2 163.33(3), C1−Ni−P1 81.41(8), C1−Ni−P2 81.96(8),
H−Ni−H 65(1), H−B−H 104(2).
bidentate ligand. The Ni···B distance of 2.207(3) Å is
comparable to those in 2a−c, but much longer than the sum
of Ni and B covalent radii of 2.08 Å.35 Disregarding the Pe
c
groups, the structure would belong to a C2v point group. The
deviation from planarity for the plane generated by P1, C1, P2,
Ni, B1, C27, and C31 atoms is only 0.0171 Å. If Y is defined as
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31P resonance of (tBuPOCOP)NiD exhibits at 219.39 ppm as a
1: 1: 1 triplet due to isotope shift and 31P−2H coupling.
Evaporation of the volatiles37 under vacuum followed by
redissolution of the residue in C6D6 shows 88% of the nickel
species containing protium on the metal, which further
supports the exchange of hydrogens between 1a and HBcat.
The fact that protium concentrates at the nickel center rather
than the boron center probably reflects an equilibrium isotope
effect, which is often generalized by the statement “deuterium
prefers the stronger bond”.38 The B−H bond dissociation
energy (BDE) of HBcat has been previously calculated as
111 kcal/mol39 whereas the BDE of a metal−hydrogen bond is
typically within the range of 55−80 kcal/mol.40
the centroid of C27 and C31 atoms, it is almost linear with Ni
and B1 with a Ni−B1−Y angle of 179.48°. These structural
parameters provide further evidence supporting the η2-
coordination mode.
Reactivities of Nickel Dihydridoborate Complex 5c. At
room temperature, a complete conversion from 5c to 1c is
accomplished under a dynamic vacuum in just 1 h. The use of
any reagents that trap either 9-BBN or 1c should also perturb
the equilibrium (similar as the one in eq 9). As predicted,
addition of 1 equiv of NEt3 to a solution of 5c in C6D6
produces 1c and 9-BBN·NEt3 in 90% conversion (Scheme 2).
Scheme 2
The first sign indicating a net reaction between 1b and HBcat
was the change of color from yellow to orange when 1 equiv of
HBcat was added to the solution of 1b in C6D6. Although the
31P{1H} NMR spectrum only shows the resonance of 1b, the
1H NMR spectrum contains a slightly broad peak at −1.90 ppm
(Δν1/2 = 28 Hz at 22 °C), a chemical shift similar to the one
observed in the 1b/9-BBN reaction. More interestingly, this
hydride resonance (for the same reaction in toluene-d8) shifts
downfield and becomes broader as the temperature decreases
(Figure 6). This can be explained by an equilibrium (eq 10)
that is fast on the NMR time scale.
Additionally, 5c readily reacts with CO2 (∼1 atm), as suggested
by an immediate color change from red to yellow. In view of
rapid insertion of CO2 into 1c, the obtained nickel species was
initially postulated as the insertion product, nickel formate
1
complex 4c. However, the H NMR spectrum of the reaction
mixture shows a new peak at 8.62 ppm, which is shifted from
8.39 ppm for the formate resonance of 4c. The 31P{1H} NMR
spectrum also displays a more downfield shifted resonance
(175.24 ppm) than the one for 4c (173.35 ppm). Perhaps the
new species is an adduct of 4c and 9-BBN through a Lewis
acid−base interaction. An alternative structure involving both a
hydride and a formate bridging nickel and boron centers cannot
be ruled out. In any event, 4c slowly forms over a period of 8 h,
with a concurrent formation of B-methoxy-9-BBN and a borate
ester (Scheme 2).
Reactions of Nickel Hydride Complexes with HBcat.
Catecholborane (HBcat) was the borane of choice when we
conducted our initial investigation of reduction (or hydro-
boration) of CO2 catalyzed by 1a.1 Compared to BH3 and
9-BBN, HBcat has a less electrophilic boron center because of
the donation of electrons from neighboring oxygen atoms. As a
result, it is less favorable for HBcat to abstract H− from a nickel
hydride complex to produce a dihydridoborate complex. When
a solution of 1a in C6D6 was treated with 1 equiv of HBcat at
room temperature, no color change was observed. Judging from
the 31P{1H} and 11B NMR spectra, there is no net reaction
Figure 6. Variable-temperature 1H NMR spectra of the 1:1 mixture of
1b and HBcat in toluene-d8.
1
between the two species. However, the H NMR spectrum
reveals the absence of NiH and BH resonances, suggesting a
rapid exchange process involving these two hydrogens. All
other resonances of 1a and HBcat remain the same as those for
individual reagents. Variable-temperature (from −50 °C to +60 °C)
experiments in toluene-d8 show negligible difference in the
1H NMR spectra.36 Similarly, the mixture of (tBuPOCOP)NiD
and HBcat (approximately 1: 1) in C6D6 shows no resonance in
the hydride region. It is interesting to note that the 31P{1H}
NMR spectrum resembles 1a rather than (tBuPOCOP)NiD.
These two species are distinguishable because the 31P
resonance of 1a appears at 219.35 ppm as a singlet while the
The lower temperature shifts the equilibrium more to the 6b
side, resulting in a significant change in chemical shift. The
line broadening reflects the decreased rates of the forward
and backward reactions; however, it is understood that even
at −70 °C the reaction remains very fast. Variable-temperature
42
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Inorganic Chemistry
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11B NMR experiments provide additional evidence supporting
the proposed equilibrium. At room temperature, only one
resonance is observed at 27.9 ppm, which corresponds to
HBcat. At −60 °C, a broad new resonance presumably for 6b
emerges at 13.7 ppm while the 11B resonance of HBcat is
substantially broadened and its intensity is diminished (see
Supporting Information). The upfield shifted 11B resonance is
more consistent with a dihydridoborate complex. In contrast,
the 11B resonance of a σ-borane complex4 or a metal boryl
complex6,9 is typically downfield from that of free borane.
Analogously, 1c and HBcat are in rapid equilibrium with a
dihydridoborate complex. The NMR data are similar to those
described above for 1b, except that more side reactions occur in
However, when 9-BBN is utilized as the borane source,
catalytic reduction of CO2 occurs readily under the influence of
nickel hydrides 1a−c (entries 4−6). As the size of the
substituents on the phosphorus donors decreases (from 1a to
1b to 1c), the required time to reach the maximum turnover
number (TON) of 100 (based on the B−H bond) becomes
longer. Arguably, electronic effects of the pincer ligand might
also be used to rationalize the reactivity differences between
these nickel hydrides. Bush and Angelici have demonstrated
t
that Bu-substituted phosphines are in general more basic than
other alkyl-substituted phosphines.41 If it is also true in our case
with the POCOP-pincer ligand, 1a could possess the most
hydridic hydrogen among the three hydrides examined, thereby
promoting the turnover-limiting insertion of boryl formate into
the nickel hydride (Cycle II in Scheme 1). Nevertheless, the
relative catalytic activity of the hydride complexes correlates
nicely with their relative reactivity observed in the stoichio-
metric reaction with 9-BBN. Complex 1a, which is the best
catalyst of the series, does not form any adduct with 9-BBN. In
contrast, complex 1b reacts with 9-BBN to form a
dihydridoborate complex 5b, as demonstrated in eq 9.
Although this process is reversible and the release of 1b from
5b is reasonably fast, the equilibrium should reduce the steady-
state concentration of the active hydride species, resulting in
slower catalysis. In the case of 1c, the analogous equilibrium is
even more favorable toward the catalytically dormant species
5c. The observation of a phosphine-borane adduct
(cPe)2PH·(9-BBN) in the stoichiometric reaction suggests
that the decomposition of catalyst 1c by 9-BBN through the
cleavage of P−O bonds may play an additional role in reducing
the concentration of 1c during catalysis.
1
this case. A small amount of H2 was detected by H NMR
spectroscopy, implying that a reaction related to eq 2 is perhaps
involved. In addition, a set of broad multiplets in the 3.8−
5.0 ppm range is likely due to the formation of (cPe)2PH·HBcat.
11B NMR spectroscopy also reveals peaks at 22.3 ppm and
14.7 ppm, which are assigned to B2(cat)3 and [B(cat)2]− based on
literature values.10a Collectively, these results once again high-
light the vulnerability of the P−O bonds in the presence of a
hydride donor if they are not sterically protected.
Catalytic Reduction of CO2 with Different Boranes.
The aforementioned reactions of nickel hydride complexes with
boranes are expected to have important implications for the
catalytic reduction of CO2. To elucidate their relationships, the
catalytic performance of different combinations of nickel
catalysts and boranes was investigated. In a typical catalytic
reaction (eq 11), a mixture of HBR′2 to hydride (in a 100:1
ratio) in C6D6 was stirred under 1 atm of CO2 atmosphere at
room temperature, and the progress of the reaction was
1
Compared to 9-BBN, HBcat has a weaker interaction with
the nickel hydride complexes. Related dihydridoborate
complexes do form when complexes 1b and 1c are mixed
with HBcat, but at room temperature they exist in such a small
quantity that they should not drastically impact the catalysis.
Experimental results (entries 7−9) show that 1b is a less active
catalyst than 1a. More surprisingly, the reaction catalyzed by 1c
did not go to completion with a TON of only 30 after 12 h. We
suspected that catalysis was plagued with severe catalyst
degradation (for 1c in particular) as described in the
stoichiometric study.
followed by H NMR spectroscopy.
As shown in Table 3, no reduction of CO2 takes place when
BH3·THF is employed as the borane source (entries 1−3).
Table 3. Catalytic Activity of Nickel Hydride Complexes in
a
the Reduction of CO2
b
entry
catalyst
borane
time (h)
TON
CONCLUDING REMARKS
1
2
3
4
5
6
7
8
9
1a
1b
1c
1a
1b
1c
1a
1b
1c
BH3·THF
BH3·THF
BH3·THF
9-BBN
24
24
24
1
0
0
■
We have synthesized or spectroscopically observed nickel
dihydridoborate complexes from the reactions between
POCOP-pincer-ligated nickel hydrides and various boranes.
The extent of formation of such complexes is not only
dependent on the substituents on the phosphorus donor atoms,
but also dependent on the type of boranes. We have shown that
nickel hydride complexes react with BH3·THF irreversibly at
room temperature to yield BH4-complexes, which can be
considered as a special case of dihydridoborate complexes.
Reversing the reactions is possible at higher temperatures if
there is a trapping agent available, or under a dynamic vacuum.
Other boranes such as 9-BBN and HBcat are also capable of
reacting with nickel hydride complexes to form the
corresponding nickel dihydridoborate complexes, unless the
hydride moiety is sterically inaccessible. However, these
reactions are reversible at room temperature, allowing nickel
hydride species to reform under most circumstances. In the
catalytic reduction of CO2 with boranes, the dihydridoborate
complexes, when they are formed, serve as dormant species
c
0
100
100
9-BBN
1.5
4
c
9-BBN
100
d
HBcat
0.75
2
100
d
HBcat
100
cd
,
HBcat
12
30
a
Reaction conditions: nickel catalyst (25 μmol), borane (2.5 mmol),
and hexamethylbenzene (62.5 μmol, internal standard) in 2 mL of
C6D6 under 1 atm of CO2 at room temperature. Based on the B−H
bond. Hexamethyldisilane was used as the internal standard. Data
from ref 3.
b
c
d
Reactions in eq 5 suggest that the nickel catalysts would be
trapped as the borohydride complexes 2a−c. At room
temperature, the release of nickel hydrides 1a−c back to the
catalytic cycles is highly unfavorable, thereby preventing the
reduction of CO2.
43
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(iPrPOCOP)Ni(BH4) (2b). 2b was prepared in 85% yield via Method
A following procedures similar to those used for 2a. This compound
was also prepared in 90% yield via Method B, although the reaction
time was shortened to 12 h. 1H NMR (400 MHz, C6D6, δ): −0.71 (br
q, NiBH4, 4H), 1.05−1.10 (m, PCH(CH3)2, 12H), 1.23−1.29 (m,
outside the catalytic cycles. The catalytic reactions are fast
when the nickel catalyst contains bulky substituents on the
phosphorus donor atoms. Another benefit of these bulky
substituents is that they protect the P−O bonds of the pincer
ligand from borane attack and thus minimize catalyst
degradation.
PCH(CH3)2, 12H), 2.07−2.14 (m, PCH(CH3)2, 4H), 6.61 (d, ArH3/5
,
3
1
3JH−H = 8.0 Hz, 2H), 6.85 (t, ArH4, JH−H = 8.0 Hz, 1H). H NMR
1
(400 MHz, CD2Cl2, δ): −1.39 (br q, JH−B = 79 Hz, NiBH4, 4H),
1.24−1.36 (m, PCH(CH3)2, 24H), 2.29−2.36 (m, PCH(CH3)2, 4H),
EXPERIMENTAL SECTION
■
6.42 (d, ArH3/5, JH−H = 8.0 Hz, 2H), 6.91 (t, ArH4, JH−H = 8.0 Hz,
1H). 13C{1H} NMR (101 MHz, C6D6, δ): 16.60 (s, CH3), 16.89 (s,
CH3), 28.35 (t, JC−P = 11.9 Hz, PCH), 105.49 (t, JC−P = 6.4 Hz,
ArC3/5), 168.75 (t, JC−P = 9.6 Hz, ArC2/6); other resonances were
obscured by the solvent peaks. 13C{1H} NMR (101 MHz, CD2Cl2, δ):
16.74 (CH3), 16.99 (t, JC−P = 2.5 Hz, CH3), 28.49 (t, JC−P = 11.9 Hz,
PC(CH3)3), 105.18 (t, JC−P = 6.5 Hz, ArC3/5), 127.92 (s, ArC4),
129.78 (t, JC−P = 21.3 Hz, ArC1), 168.52 (t, JC−P = 9.5 Hz, ArC2/6).
31P{1H} NMR (162 MHz, C6D6, δ): 199.43 (s). 31P{1H} NMR (162
MHz, CD2Cl2, δ): 199.28 (s). 11B NMR (128 MHz, C6D6, δ): −33.5
3
3
General Comments. Unless otherwise noted, all the organo-
metallic compounds were prepared and handled under an argon
atmosphere using standard Schlenk and inert-atmosphere box
techniques. Dry and oxygen-free solvents (toluene and tetrahydrofuran
(THF)) were collected from an Innovative Technology solvent
purification system and used throughout the experiments. Benzene-d6
and toluene-d8 were distilled from Na and benzophenone under an
argon atmosphere. Other solvents (pentane and CD2Cl2) were used as
received from commercial sources. Catecholborane was purified by
vacuum distillation prior to use, and other boranes were purchased
from Sigma-Aldrich and used without further purification. 1H, 13C, and
31P NMR spectra were recorded on a Bruker Avance-400 MHz
spectrometer. 11B NMR spectra were recorded on a Bruker AMX
(quin, JB−H = 63.9 Hz). 11B{1H} NMR (128 MHz, C6D6, δ): −33.5
1
(s). Selected data from ATR-IR (solid, cm−1): 2406 (s), 2382 (s),
2274 (w), 1947 (br), 1144 (s). Selected data from transmission-IR
(CH2Cl2, cm−1): 2398 (s), 2380 (s), 2273 (w), 1954 (br), 1143 (s).
Anal. Calcd for C18H35O2BP2Ni: C, 52.10; H, 8.50. Found: C, 52.30;
H, 8.47.
400 MHz wide-bore spectrometer. Chemical shift values in 1H and 13
C
NMR spectra were referenced internally to the residual solvent
resonances. 31P and 11B NMR spectra were referenced externally to
85% H3PO4 (0 ppm) and BF3·Et2O (0 ppm), respectively. Infrared
spectra were recorded on a Thermo Scientific Nicolet 6700 FT-IR
spectrometer equipped with smart orbit diamond attenuated total
reflectance (ATR) accessory. (tBuPOCOP)NiH (1a),18 (iPrPOCOP)-
(
cPePOCOP)Ni(BH4) (2c). 2c was prepared in 83% yield (via Method
A) and 79% yield (via Method B) by procedures similar to those used
1
for 2b. H NMR (400 MHz, C6D6, δ): −0.63 (br d, NiBH4, 4H),
1.37−2.10 (m, CH2, 32H), 2.34−2.38 (m, PCH, 4H), 6.64 (d, ArH3/5
,
3
1
3JH−H = 7.2 Hz, 2H), 6.87 (t, ArH4, JH−H = 7.2 Hz, 1H). H NMR
(400 MHz, CD2Cl2, δ): −1.39 (br q, NiBH4, 4H), 1.61−1.77 (m, CH2,
32H), 2.41−2.46 (m, PCH, 4H), 6.39 (d, ArH3/5, 3JH−H = 8.0 Hz, 2H),
NiH (1b),18
( (
cPePOCOP)NiH (1c),3 tBuPOCOP)NiCl (3a),18
(iPrPOCOP)NiCl (3b),42 and (cPePOCOP)NiCl (3c)3 were prepared
as described in the literature. (tBuPOCOP)NiD (98 atom % D) was
synthesized from 3a and LiAlD4 (98 atom % D) following a procedure
used for 1a.18
3
6.89 (t, ArH4, JH−H = 8.0 Hz, 1H). 13C{1H} NMR (101 MHz, C6D6,
δ): 26.57 (t, JC−P = 4.4 Hz, CH3), 26.99 (t, JC−P = 3.6 Hz, CH3), 27.88
(t, JC−P = 3.0 Hz, CH3), 28.32 (CH3), 39.55 (t, JC−P = 13.0 Hz, PCH),
105.58 (t, JC−P = 6.5 Hz, ArC3/5), 130.60 (t, JC−P = 21.5 Hz, ArC1),
168.53 (t, JC−P = 9.9 Hz, ArC2/6); one resonance was obscured by the
Synthesis of (tBuPOCOP)Ni(BH4) (2a). Method A from 1a: Under
an argon atmosphere, a 1.0 M solution of BH3·THF in THF (0.33 mL,
0.33 mmol) was added slowly to a Schlenk flask containing a chilled
solution (0 °C) of 1a (100 mg, 0.22 mmol) in 10 mL of toluene. The
color of the solution changed immediately from yellow to orange.
After stirring at room temperature for 30 min, the reaction mixture was
exposed to air and filtered through a pad of Celite. Removal of the
volatiles under vacuum afforded an orange solid, which was further
purified by washing with pentane (3 mL × 3) and then drying under
vacuum (91 mg, 88% yield). Method B from 3a: Under an argon
atmosphere, 10 mL of toluene was added to a Schlenk flask containing
3a (100 mg, 0.20 mmol) and NaBH4 (9.0 mg, 0.24 mmol). The
reaction mixture was stirred at room temperature for 3 d, resulting in a
yellow-orange solution, which was filtered through a pad of Celite (in
air) and evaporated to dryness. The obtained orange solid was washed
with pentane (3 mL × 3) and dried under vacuum to afford the pure
solvent peaks. 13C{1H} NMR (101 MHz, CD2Cl2, δ): 26.67 (t, JC−P
=
4.5 Hz, CH3), 27.07 (t, JC−P = 3.7 Hz, CH3), 27.85 (t, JC−P = 3.2 Hz,
CH3), 28.36 (CH3), 39.49 (t, JC−P = 13.1 Hz, PCH), 105.19 (t, JC−P
=
6.4 Hz, ArC3/5), 127.80 (s, ArC4), 130.17 (t, JC−P = 21.3 Hz, ArC1),
168.22 (t, JC−P = 9.7 Hz, ArC2/6). 31P{1H} NMR (162 MHz, C6D6, δ):
190.42 (s). 31P{1H} NMR (162 MHz, CD2Cl2, δ): 190.43 (s). 11B
NMR (128 MHz, C6D6, δ): −32.2 (br). 11B{1H} NMR (128 MHz,
C6D6, δ): −32.2 (s). Selected data from ATR-IR (solid, cm−1): 2387
(s), 2370 (s), 2068 (br), 1886 (w), 1799 (br), 1122(s). Selected data
from transmission-IR (CH2Cl2, cm−1): 2381 (br s), 2075 (br), 1885
(br), 1792 (br), 1123(s). Anal. Calcd for C26H43O2BP2Ni: C, 60.16;
H, 8.35. Found: C, 60.02; H, 8.51.
Formation of (cPePOCOP)Ni(BC8H16) (5c). Under an argon
atmosphere, 9-BBN dimer (13.3 mg, 54.5 μmol) and 1c (50 mg,
99 μmol) were mixed with ∼1 mL of toluene in a scintillation vial. After
standing at room temperature for 1 h, the resulting red solution was
carefully layered with about the same volume of pentane and then kept
in a −30 °C freezer for at least 12 h. Red crystals obtained were suitable
for X-ray crystallographic study. The NMR sample was prepared by
mixing a 1:2 ratio of 9-BBN dimer and 1c in ∼0.6 mL of C6D6. About 5%
of the total nickel species was identified as unreacted 1c. When the
solution was kept for several hours, a new species started to form and was
product (68 mg, 71% yield). 1H NMR (400 MHz, C6D6, δ): −0.45 (br
3
q, NiBH4, 4H), 1.37 (vt, JH−P = 7.0 Hz, CH3, 36H), 6.56 (d, ArH3/5
,
3JH−H = 8.0 Hz, 2H), 6.85 (t, ArH4, JH−H = 8.0 Hz, 1H). H NMR
(400 MHz, CD2Cl2, δ): −1.13 (br q, 1JH−B = 80 Hz, NiBH4, 4H), 1.43
(vt, 3JH−P = 7.0 Hz, CH3, 36H), 6.41 (d, ArH3/5, 3JH−H = 8.0 Hz, 2H),
3
1
6.90 (t, ArH4, JH−H = 8.0 Hz, 1H). 13C{1H} NMR (101 MHz, C6D6,
3
δ): 27.95 (t, JC−P = 2.7 Hz, CH3), 40.06 (t, JC−P = 7.6 Hz, PC(CH3)3),
105.14 (t, JC−P = 6.1 Hz, ArC3/5), 169.47 (t, JC−P = 8.8 Hz, ArC2/6);
other resonances were obscured by the solvent peaks. 13C{1H} NMR
(101 MHz, CD2Cl2, δ): 28.03 (s, CH3), 40.28 (t, JC−P = 7.7 Hz,
PC(CH3)3), 104.86 (t, JC−P = 6.1 Hz, ArC3/5), 127.65 (t, JC−P = 20.5
Hz, ArC1), 127.76 (s, ArC4), 169.34 (t, JC−P = 8.7 Hz, ArC2/6). 31P{1H}
NMR (162 MHz, C6D6, δ): 200.15 (s). 31P{1H} NMR (162 MHz,
CD2Cl2, δ): 200.03 (s). 11B NMR (128 MHz, C6D6, δ): −35.9 (quin,
1JB−H = 65.4 Hz). 11B{1H} NMR (128 MHz, C6D6, δ): −35.9 (s).
Selected data from ATR-IR (solid, cm−1): 2412 (s), 2397 (s), 1961
(br). Selected data from transmission-IR (CH2Cl2, cm−1): 2412 (s),
2397 (s), 1955 (br). Anal. Calcd for C22H43O2BP2Ni: C, 56.10; H,
9.20. Found: C, 55.88; H, 9.15.
1
identified as (cPe)2PH·(9-BBN). Characterization data for 5c: H NMR
(400 MHz, C6D6, δ): −1.80 (br s, Ni(H)2B, 2H), 0.91 (br, BCH, 2H),
1.36−2.35 (m, CH and CH2, 48H), 6.60 (d, ArH3/5, 3JH−H = 8.0 Hz, 2H),
6.83 (t, ArH4, 3JH−H = 8.0 Hz, 1H). 31P{1H} NMR (162 MHz, C6D6, δ):
187.72 (s). 11B NMR (128 MHz, C6D6, δ): −11.4 (br s). 11B{1H} NMR
(128 MHz, C6D6, δ): −11.3 (s). Selected data from transmission-IR
(toluene, cm−1): 1943 (w), 1857 (w).
Phosphine-Borane Adduct (cPe)2PH·(9-BBN) Generated in
Situ. The NMR sample was prepared by mixing a 2:1 ratio of
(cPe)2PH and 9-BBN dimer in ∼0.6 mL of toluene-d6. 1H NMR
(400 MHz, C7D8, δ): 1.23−2.32 (m, CH and CH2, 32H), 4.14 (ddt, 1JP−H
=
44
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Table 4. Crystal Data Collection and Refinement Parameters
2a
2b
2c
5c
empirical formula
crystal system
space group
a, Å
C22H43O2BP2Ni
triclinic
C18H35O2BP2Ni
monoclinic
P21/c
C26H43O2BP2Ni
triclinic
C34H55O2BP2Ni
monoclinic
P21/c
P1
̅
P1
̅
8.2870(2)
12.1490(3)
13.4267(3)
100.274(1)
95.653(1)
105.074(2)
1269.32(5)
2
12.9681(4)
23.5433(8)
14.6846(5)
90
8.7254(1)
10.5060(2)
14.5596(2)
81.742(1)
86.572(1)
83.280(1)
1310.50(3)
2
15.7800(7)
13.0913(6)
17.1942(8)
90
b, Å
c, Å
α, deg
β, deg
100.352(1)
90
112.518(2)
90
γ, deg
volume, Å3
4410.4(3)
8
3281.2(3)
4
Z
no. of data collected
no. of unique data, Rint
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
11009
33917
11353
22642
4390, 0.0327
0.0410, 0.1045
0.0522, 0.1119
6774, 0.0987
0.0498, 0.1124
0.0844, 0.1286
4546, 0.0244
0.0389, 0.0999
0.0452, 0.1043
5826, 0.0571
0.0470, 0.1179
0.0728, 0.1291
3
323.6 Hz, JH−H = 15.2 Hz, JH−H = 3.6 Hz, P−H, 1H); BH was not
located. 31P{1H} NMR (162 MHz, C7D8, δ): −6.80 (s). 11B NMR
(128 MHz, C7D8, δ): −18.1 (s).
hexamethylbenzene (10.0 mg, 62.5 μmol) was added as an internal
standard except for the reactions involving 1c. To avoid the overlap of
proton resonances between hexamethylbenzene and 1c, hexamethyl-
disilane was used as the internal standard. The mixture was degassed
by a freeze−pump−thaw cycle and placed under 1 atm of CO2. At
different time intervals, a small portion of the clear liquid layer
(ca. 0.6 mL) was withdrawn from the flask and transferred into a
Release of BH3 from 2a−c under Vacuum. A solid sample of a
nickel borohydride complex (25 μmol) in a J. Young NMR tube was
heated at 60 °C under a dynamic vacuum for 24 h. The residue was
dissolved in about 0.6 mL of benzene-d6 for NMR analysis. The
percentage conversion for each reaction was calculated by comparing
the integration of the produced nickel hydride species with that of
unreacted borohydride complex from the 31P NMR spectrum.
Reactions of 2a−c with Triethylamine. A screw cap NMR
tube was charged with a solution of a nickel borohydride complex
(12 μmol) in 0.6 mL of benzene-d6. Triethylamine (1.7 μL, 12 μmol)
was added via a microliter syringe, and the NMR tube was heated by a
60 °C oil bath. The progress of the reaction was monitored by 1H and
31P{1H} NMR spectroscopy. The percentage conversion for each
reaction was calculated by comparing the integration of the produced
nickel hydride species with that of unreacted borohydride complex
from the 31P NMR spectrum.
1
J. Young NMR tube under an argon atmosphere. From the H NMR
spectrum of the aliquot, turnover number (TON) was calculated by
comparing the integration of CH3OBR′2 methyl resonance (near 3.3 ppm)
with that of the internal standard (2.12 ppm).
X-ray Structure Determination. Single crystals of 2a−c were
grown from pentane at −30 °C. Single crystals of 5c were grown from
toluene-pentane at −30 °C. Crystal data collection and refinement
parameters are summarized in Table 4. Intensity data were collected at
150 K on a Bruker SMART6000 CCD diffractometer using graphite-
monochromated Cu Kα radiation, λ = 1.54178 Å. The data frames
were processed using the program SAINT. The data were corrected
for decay, Lorentz, and polarization effects as well as absorption and
beam corrections based on the multiscan technique. The structures
were solved by a combination of direct methods in SHELXTL and the
difference Fourier technique and refined by full-matrix least-squares
procedures. Non-hydrogen atoms were refined with anisotropic
displacement parameters. H-atoms attached to the boron were located
directly from the difference map, and their positions refined. All
remaining H-atom positions were calculated and treated with a riding
model. None of the structures has solvent present in the crystalline
lattice. The cyclopentyl rings in 2c and 5c show typical disorder; a
two-component model is presented for C24/C25 in 2c (major
occupancy is 55%) and C25 in 5c (major occupancy is 70%).
Compound 2b crystallized as two independent molecules in the
crystalline lattice.
Reactions of 2a−c with CO2. In a glovebox, a nickel borohydride
complex (25 μmol) and 0.6 mL of benzene-d6 were transferred into a
J. Young NMR tube. The resulting solution was degassed by
performing three freeze−pump−thaw cycles and then placed under
1 atm of CO2. The NMR tube was heated by a 60 °C oil bath, and the
progress of the reaction was monitored by H and 31P{1H} NMR
1
spectroscopy. The percentage conversion for each reaction was
calculated by comparing the integration of the produced nickel
formate complex with that of unreacted borohydride complex from the
31P NMR spectrum.
Reaction of 5c with Triethylamine. In a J. Young NMR tube,
complex 5c was generated in situ by mixing 1c (10.0 mg, 19.8 μmol)
and 9-BBN dimer (2.4 mg, 9.9 μmol) in 0.6 mL of benzene-d6 at
room temperature for 1 h. To this solution, triethylamine (2.8 μL,
19.8 μmol) was added via a microliter syringe. A complete conversion
to 1c was accomplished within 30 min, as confirmed by 1H and 31P NMR
spectroscopy.
Reaction of 5c with CO2. In a J. Young NMR tube, complex 5c
was generated in situ by mixing 1c (10.0 mg, 19.8 μmol) and 9-BBN
dimer (2.4 mg, 9.9 μmol) in 0.6 mL of benzene-d6 at room
temperature for 1 h. The resulting solution was degassed by
performing three freeze−pump−thaw cycles and then placed under
1 atm of CO2. The progress of the reaction was monitored by 1H and
31P NMR spectroscopy. After 8 h, 4c was the only nickel species
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data in CIF format and NMR data in
PDF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
observed by 31P{1H} spectroscopy. The boron-containing products
were identified as the known B-methoxy-9-BBN43 and a borate ester.44
Catalytic Reduction of CO2 with Boranes (BH3·THF, 9-BBN,
and HBcat). Under an argon atmosphere, a flame-dried 50 mL
Schlenk flask was charged with a nickel hydride complex (25 μmol), a
borane (272 μL, 2.50 mmol), and 2 mL of C6D6. To this solution,
Corresponding Author
*E-mail: hairong.guan@uc.edu.
Notes
The authors declare no competing financial interest.
45
dx.doi.org/10.1021/ic300587b | Inorg. Chem. 2013, 52, 37−47

Inorganic Chemistry
Article
(13) Kuklin, S. A.; Sheloumov, A. M.; Dolgushin, F. M.; Ezernitskaya,
M. G.; Peregudov, A. S.; Petrovskii, P. V.; Koridze, A. A.
Organometallics 2006, 25, 5466−5476.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (CHE-0952083)
and the donors of the American Chemical Society Petroleum
Research Fund (49646-DNI3) for support of this research. We
are also grateful to Dr. Keyang Ding for his assistance with
NMR experiments. S.C. thanks the University of Cincinnati
Sigma Xi Chapter for a Grants-in-Aid of Research award. X-ray
data were collected on a Bruker SMART6000 diffractometer
which was funded through an NSF-MRI grant (CHE-
0215950).
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